1. Field of the Invention
The present invention relates to methods and devices providing holographic gratings which reflect at infrared wavelengths in photorefractive materials such as lithium niobate (LiNbO.sub.3). Materials such as LiNbO.sub.3 have little or no photorefractive sensitivity in the infrared (IR), which rules out direct writing of holographic gratings using infrared wavelengths. However, there are many potential applications for holographic optical elements such as filters and wavelength-selective mirrors at these wavelengths. Some of these applications include narrowband IR imaging filters, holographic reflectors for external cavity lasers, holographic multiplexers and demultiplexers for wavelength division multiplexing (WDM) communication systems, and wavelength-selective elements for optical spectrum analyzers and tuners. This is desirable to provide photorefractive devices which can store, filter or modify signals at infrared wavelengths.
2. Description of Related Art
As referenced in the patent applications cited above, photorefractive materials have been known to be effective media for storing volume holographic gratings. One application of this technology is extremely narrow bandwidth reflection filters using volume holographic gratings recorded in these materials. Holographic grating filters with 0.12 .ANG. fabricated at 6562.8 .ANG. (the solar H.sub..alpha. absorption wavelength) Full Width at Half Maximum (FWHM) bandwidths have been using this technology as described in the parent patent applications, supra. Similar filters, particularly those with their center wavelengths in the IR band, have been proposed to characterize and control the output wavelengths of semiconductor lasers for optical telecommunications applications.
In addition to their narrow bandwidth properties, holographic gratings written in some photorefractive materials, such as LiNbO.sub.3, have the additional feature of being convertible to permanent, i.e., fixed, gratings that are not erased by illumination (J. J. Amodei and D. L. Staebler, "Holographic recording in lithium niobate," RCA Review vol. 33, pp. 71-94 (1972)). Through an enhanced fixing method described in the parent patent applications, Supra, high fixed diffraction efficiencies are obtained for holographic gratings in LiNbO.sub.3, making devices using these gratings feasible for commercial applications.
However, many photorefractive materials such as LiNbO.sub.3 have little or no photosensitivity at wavelengths greater than about 700 nm, although most optical telecommunications systems operate well above this range. Infrared wavelength bands centered around 1.31 .mu.m and 1.55 .mu.m are employed very widely in optical telecommunication applications because they are within the transmission windows of optical fibers and the gain windows of optical amplifiers. Holographic gratings at these wavelength bands can provide essential elements for components such as accurate wavelength lasers, narrow bandwidth filters, and optical tuners for telecommunications systems. In addition, holographic gratings that are reflective in the IR can be important for such applications as optical spectrum analyzers for testing telecommunications components and systems, spectroscopy, and remote sensing in the infrared band.